1. Field of the Invention
The present invention generally relates to a method for forming a semiconductor light emitting diode, and in particular to a method for forming a compound semiconductor light emitting diode.
2. Description of the Prior Art
Light emitting diodes using a double heterostructure InGaAlP have been demonstrated in recent year. A typical double heterostructure InGaAlP device has a GaAs n type substrate on which several epitaxial layers are grown to form the light emitting diode. The InGaAlP-based alloy is an important semiconductor system for the fabrication of light emitting diode (LED) with very high luminescence emission at a wavelength between red and green region. The In0.5(Ga1xe2x88x92xAlx)0.5P alloy is lattice matched to the GaAs substrate and has a direct transition of the bandgap with an energy range from 1.9 eV to around 2.3 eV with the Al composition of 0 less than x less than 0.7, where x designates the mole fraction of aluminum. The band gap of the In0.5(Ga1xe2x88x92xAlx)0.5P alloy is indirect with a band gap energy range of 2.3 eV for xxcx9c0.7 and 2.35 eV for xxcx9c1.
For efficient light emission, one needs to work in the direct bandgap with a strong radiative recombination of carriers and high efficiency of light emitting. The InGaAlP-based LED with the shorter emission wavelengths between red and yellow-green visible color has a direct transition for the high brightness light emission. In addition, the In0.5(Ga1xe2x88x92xAlx)0.5P alloy has a nearly perfect lattice alignment and is charge balance to the GaAs semiconductor substrate at the III-V/III-V interface which represents a good candidate for the epitaxial growth in an atomic-level, like precise control on the thickness and composition of the multiple quantum well (MQW). This leads to a good material quality of the heterostructure and epitaxial feasibility for a complicated and delicated device structure. Therefore, the quaternary In0.5(Ga1xe2x88x92xAlx)0.5P alloy system attracts a great attention for the fabrication of high performance visible light-emitting diodes to improve the efficiency of light emitting diodes.
FIG. 1 shows a schematic diagram of a conventional device structure of a light emitting diode. In this figure, the device structure comprises a double heterostructure (DH) with the quaternary In0.5(Ga1xe2x88x92xAlx)0.5P alloy system grown on a n type GaAs substrate 101. The DH is constructed by an n-type In0.5(Ga1xe2x88x92xAlx)0.5P lower cladding layer 102, an undoped In0.5(Ga1xe2x88x92xAlx)0.5P active layer 103, a p-type In0.5(Ga1xe2x88x92xAlx)0.5P upper cladding layer 104, a p-type GaP or p-type AlGaAs current spreading layer 105, a top metal contact 106, and a bottom metal contact 107.
In FIG. 1, the LED is a p-n junction with a forward bias to inject holes from a p-type cladding layer 104 and electrons from a n-type cladding layer 102 into an active region 103. The active layer 103 emits visible light due to the recombination of the electrons and holes in this region. Electrons and holes are injected as minority carriers across the active region 103 and they recombine either by radiative recombination or non-radiative recombination. The emitting wavelength of the InGaAlP-based LED can be adjusted by changing the Al composition of the In0.5(Ga1xe2x88x92xAlx)0.5P alloy in the active layer 103, having a right energy gap to meet a specific wavelength of emission light. For instance, a shorter wavelength such as in yellow or yellow-green color requires a higher Al composition in the In0.5(Ga1xe2x88x92xAlx)0.5P active layer 103 for light emission. The thickness of the active layer 103 is critical, and is normally less than the injected carrier diffusion length for the carrier recombination. The efficiency of the light emission is reduced in a thick active region due to a low carrier density. A typical thickness of the active region is around 0.3 to 0.5 xcexcm. The active region is an area for the carrier injection and recombination to generate light. The requirement on material quality in the active region is very high for achieving a high efficient light emission. This requires a very low background of intrinsic impurity in the active region which may reduce the concentration of nonradiative recombination center. A high doping background of the active region is mainly contributed from a high density of deep traps in the active region which may cause nonradiative recombination in the process of light emission. A clean and low impurity reaction in the reaction chamber is essential for the growth of the active region. Typically, the In0.5(Ga1xe2x88x92xAlx)0.5P active layer 103 is an undoped layer, either n or p-type, with a doping concentration of 5*1015 to 1*1017/cm2. On the other hand, the background of the doping level is increased with an increase in the composition of Al in the active region. This is due to an increase on the impurity level at a higher Al concentration in the active region. For a shorter emission wavelength, therefore, the increase of Al composition in the active region associates with a reduction on the internal quantum efficiency of emission light. As described above, a higher Al concentration in the active region associates with an increase on the deep level causing non-radiative recombination in the light emitting layer that decrease the efficiency of the light emission.
The n-type and p-type cladding layers provide a source of injection carriers and have an energy gap higher than that of the active layer 103 for the confinement of the injecting carriers and emitting light. These cladding layers require a good conductivity and suitable doping concentration to supply enough injected carriers into the active region to achieve a high efficiency in light emission. The thickness of the In0.5(Ga1xe2x88x92xAlx)0.5P layer 102 should be thick enough to prevent the carriers in the active region from flowing back to the cladding layers, but not too thick to affect the emission efficiency of the LED. As a result, a large portion of injected carriers overflow into the cladding layers, and current leakage occurs due to the non-radiative recombination of these overflow carriers. Consequently, the radiation efficiency in the conventional LED containing double heterostructure (DH) degrades as the wavelength of the device becoming shorter.
Following the p-type cladding layer 104, there is a current diffusion layer 105 for spreading out the emitting light efficiently. The current spreading layer 105 requires a semiconductor to be transparent to the wavelength of the emission light from the active region. The previous discussions are the prior art structure of the traditional light emitting diodes. In addition, the window layer needs to spread current efficiently into the active layer and cladding layer which requires a high doping level and a thick window layer.
To overcome the problem mentioned above, the LED must be designed functionally so that the emission light can be extracted out of the light emitting diode as much as possible to increase the light efficiency. In this invention, several claims in the InGaAlP-based LED are listed below for fabricating an efficient light emitting diode.
It is an object of the invention to provide a method for manufacturing a high-efficiency light emitting diode;
Because the energy bands within the material depend on the material and its doping, the energy transition, and thus the color of the radiation it produces, is limited by the well-known relationship (E=hv) between the energy (E) of a transition and the frequency (v) of the light it produces.
The present invention provides a method to emphasize the growing process such as the AlGaAs-based light re-emitting layer, the InGaAlP-based light emitting layer and the GaP- AlGaP or AlGaAs-based window layer are grown epitaxially by Organometallic Vapor-Phase Epitaxy (OMVPE) on a tilted GaAs substrate with a misorientation toward  less than 111 greater than A with a wavelength between 560 and 650 nm.
In addition, the insertion of an electron reflector layer containing Iny(Ga1xe2x88x92xAlx)1xe2x88x92yP/In0.5(Ga1xe2x88x92xAlx)0.5P superlattice structure, an InGaAlP-based lattice gradient layer for the improvement of quantum efficiency of light emission, and film quality of In0.5(Ga1xe2x88x92xAlx)0.5P/GaP heterostructure are also utilized to the light emitting diode.
The efficiency of the light emitting diode also depends on the alignment of p-n junction which is related to the doping levels and profiles of the n-type and p-type cladding layers. A gradient doping profile or a doping profile with a lower doping level near the multiple quantum well (MQW) and a higher doping level away from the MQW for a better alignment of the p-n junction are also proposed in this invention.
Furthermore, a 0.2-0.6% tensile stress in the MQW is claimed in this invention for a better efficiency of the light emitting diode.